1. Field of the Invention PA1 a light source unit including the point light source and at least one optical element for forming an intermediate image of the point light source, the point light source and the optical element being replaceable as a unit; and PA1 an optical system receiving the intermediate image for forming a final image plane at a prescribed position; PA1 wherein an absolute value m of a lateral magnification of the intermediate image with respect to the point light source satisfies the relation m&gt;1.
The present invention relates to optical devices and more particularly to an optical device which focuses a luminous flux emitted from a point light source into an image at a prescribed position to perform prescribed optical scanning, such as an image recording device.
2. Background of the Invention
In various optical devices using point light sources such as laser diodes, the light source may break down for some reason or other before its product lifetime is expired. If a light source breaks down, the light source must be replaced, but when the focal depth of the final image plane is shallow, a shift between the light source positions before and after replacement will become a problem.
FIG. 1 is a diagram conceptually showing an optical system in a conventional optical device (for example, refer to Japanese Patent Laying-Open No.1-231015.) In FIG. 1, if the NA (hereinafter, an abbreviation for numerical aperture) of the emitted luminous flux on the light source 1 side is designated as NA.sub.OBJ., and if the numerical aperture of the incident luminous flux on the final image plane side is designated as NA.sub.IMG., and if the positional shift of the light source 1 in the optical axis direction before and after replacement is designated as .DELTA.Z.sub.1, then the moving distance .DELTA.Z.sub.2 ' of the final image plane in the optical axis direction is represented by the equation (1) shown below. EQU .DELTA.Z.sub.2 =(NA.sub.OBJ./NA.sub.IMG.).sup.2 .times..DELTA.Z.sub.1( 1)
Accordingly, if the focal depth of the final image plane is .delta., then the relation EQU (NA.sub.OBJ./NA.sub.IMG.).sup.2 .times..DELTA.Z.sub.1 &lt;.delta.
must be satisfied to obtain a clear optical image.
Generally, it is required that the numerical aperture NA.sub.OBJ. on the light source side be as large as possible. This is due to the fact that a larger numerical aperture NA.sub.OBJ. increases a captured amount of the emitted light intensity of the light source 1 to enhance the optical utilization efficiency. If the optical utilization efficiency of the light source is enhanced, a required light intensity can be obtained on the final image forming plane side even with a low power light source. In the case of conventional optical equipment, the numerical aperture NA.sub.IMG. on the final image plane side is almost determined by the specification of the equipment. In the case of a scanning optical system, for example, the scanning line pitch determines the image forming beam diameter, which uniquely determines the numerical aperture NA.sub.IMG.
As described above, the numerical aperture NA.sub.OBJ. on the light source side is preferably as large as possible, but as can be seen from equation (1), the moving distance .DELTA.Z.sub.2 ' of the final image plane when the light source is replaced increases in proportion to a square of the numerical aperture NA.sub.OBJ. Accordingly, there has generally been a problem that the numerical aperture NA.sub.OBJ. on the light source side cannot be indiscriminately increased. If the numerical aperture NA.sub.OBJ. is increased to enhance the optical utilization efficiency of the light source, high accuracy in positioning the light source is required when replacing the light source. This requires positioning by the use of component parts worked with higher accuracy or positioning by more difficult adjustments, which will result in a high price, despite replacement parts, or will cause problems of time-consuming adjustment etc.